2network models

8/13/2019 2network Models

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Network Models


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2.2

Tasks involved in sending a letter


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Communication Architecture

Strategy for connecting host computers and othercommunicating equipment.

Defines necessary elements for data communication between devices.

A communication architecture, therefore, defines astandard for the communicating hosts.

A programmer formats data in a manner defined by thecommunication architecture and passes it on to thecommunication software .

Separating communication functions adds flexibility, forexample, we do not need to modify the entire hostsoftware to include more communication devices.


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Layer ArchitectureLayer architecture simplifies the networkdesign.It is easy to debug network applications in alayered architecture network.

The network management is easier due to thelayered architecture.

Network layers follow a set of rules , called

protocol.The protocol defines the format of the data

being exchanged, and the control and timingfor the handshake between layers.


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International Standards Organization ( ISO )- a multinational body dedicated to

worldwide agreement on internationalstandards.Open Systems Interconnection ( OSI ) model

- An ISO standard that covers all aspectsof network communications

- introduced in the late 1970s.

ISO is the organization.

OSI is the model.

Note


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OSI Reference Modelconsidered the primary Architectural model for inter-

computer communications.describes how information or data makes its way fromapplication programmes (such as spreadsheets) through anetwork medium (such as wire) to another application

programme located on another network.

divides the problem of moving information betweencomputers over a network medium into SEVEN smaller

and more manageable problems .This separation into smaller more manageable functionsis known as layering.


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OSI Reference Model: 7 LayersOSI Model


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Layer Responsible For:

7.) Application Provides Services to User Apps

6.) Presentation Data Representation

5.) Session Communication Between Hosts

4.) Transport Flow Ctrl, Error Detection/Correction

3.) Network End to End Delivery, Logical Addr

2.) Data Link Media Access Ctrl, Physical Addr

1.) Physical Medium, Interfaces, Puts Bits on Med.Unreliable Wire

Reliable Across Physical Line

Unreliable Thru Multi-Node Network

Reliable End to End

Sync Points and Dialogs

& Syntax

User Level Processing


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Layer Example

7.) Application HTTP, FTP, SMTP

6.) Presentation ASCII, JPEG, PGP

5.) Session BOOTP, NetBIOS, DHCP, DNS

4.) Transport TCP, UDP, SPX

3.) Network IP, IPX, ICMP

2.) Data Link Ethernet, Token Ring, Frame Relay

1.) Physical Bits, Interfaces, Hubs


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2.10

TCP/I P and OSI model


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OSI: A Layered Network ModelThe process of breaking up the functions or tasks ofnetworking into layers reduces complexity.

Each layer provides a service to the layer above it in the protocol specification.

Each layer communicates with the same layers

software or hardware on other computers.Lower 4 layers are concerned with the flow of data fromend to end through the network.

Upper three layers of the OSI model are oriented moretoward services to the applications.Data is Encapsulated with the necessary protocolinformation as it moves down the layers before networktransit.


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Internet Model

Peer-to-Peer Processes

Functions of Layers

Summary of Layers


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2.13

Figure 2.3 The interaction between l ayers in the OSI model


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2.15

Physical layer


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Data Link Layer

Data link layer attempts to provide reliablecommunication over the physical layer interface.Breaks the outgoing data into frames and reassemble the

received frames.Create and detect frame boundaries.Handle errors by implementing an acknowledgement andretransmission scheme.Implement flow control .Supports points-to-point as well as broadcastcommunication.

Supports simplex, half-duplex or full-duplexcommunication.

OSI Model


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2.17

Data link layer


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2.18

H op-to-hop del ivery


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2.21

Sour ce-to-destination delivery


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Transport Layer provide a reliable mechanism for the exchange ofdata between two processes in different computers.

Ensures that the data units are delivered error free .

Ensures that data units are delivered in sequence .

Ensures that there is no loss or duplication of dataunits.

Provides connectionless or connection orientedservice.Provides for the connection management .

Multiplex multiple connection over a singlechannel.

OSI Model


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2.23

Transpor t layer


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OS d l


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Session LayerProvides mechanism for controlling the dialogue

between the two end systems. It defines how to start,control and end conversations (called sessions)

between applications.requests for a logical connection to be established onan end- users request.log-on or password validation is also handled by thislayer.responsible for terminating the connection .

provides services like dialogue discipline which can be full duplex or half duplex. provide check-pointing mechanism such that if a

failure of some sort occurs between checkpoints, alldata can be retransmitted from the last checkpoint.

OSI Model


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d l


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Presentation Layer

defines the format in which the data is to beexchanged between the two communicatingentities.

handles data compression and data encryption(cryptography).

OSI Model


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2.28

Presentation layer

OSI M d l


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Application Layer

interacts with application programs

the highest level of OSI model.contains management functions tosupport distributed applications.

Examples of applications are filetransfer, electronic mail, remote loginetc.

OSI Model


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2.30

Application layer

OSI Model


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OSI in Actionmessage begins at the top

application layer and movesdown the OSI layers to the bottom physical layer.

the message descends, each

successive OSI model layeradds a header to it.

A header is layer-specificinformation that basically

explains what functions thelayer carried out.

Conversely, at the receivingend, headers are striped from

the message as it travels upthe corresponding layers.

OSI Model

OSI Model


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OSI in Action

OSI Model


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2.33

Figure 2.4 An exchange using the OSI model

TCP/IP Model


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OSI & TCP/IP Models

TCP/IP Model


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2.35

Addresses in TCP/I P


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2.37

I n F igure 2.19 a node with physical address 10 sends aframe to a node with physical address 87. The two nodesare connected by a link (bus topology LAN). As thef igure shows, the computer with physical address 10 isthe sender, and the computer with physical address 87 isthe receiver.

Example 2.1


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2.38

Physical addresses


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2.39

M ost local-area networks use a 48-bit (6-byte) physicaladdress wr itten as 12 hexadecimal digits; every byte (2hexadecimal digits) i s separated by a colon, as shownbelow:

Example 2.2

07:01:02:01:2C:4B

A 6-byte (12 hexadecimal digits) physical address.


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2.41

F igure 2.20 shows a part of an internet with two routersconnecting three LANs. Each device (computer orrouter ) has a pair of addresses (logical and physical) f oreach connection. I n this case, each computer isconnected to only one l ink and therefore has only onepair of addresses. Each router, however, is connected tothree networks (onl y two are shown in the f igur e). So

each r outer has three pairs of addresses, one for eachconnection.

Example 2.3


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2.42

Figure 2.21 Por t addr esses


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Example 2.5

A por t address is a 16-bit address represented by onedecimal number as shown.

753

A 16-bit port address representedas one single number.


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2.45

Summary of layers


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